1. Technical Field
The present disclosure relates to lasers and more specifically to a side pumped, passively Q-switched monolithic ring laser.
2. Introduction
In recent years researchers have devoted considerable effort to develop compact, long-lifetime, and reliable diode-pumped solid state lasers for a variety of applications, including use in space. The National Aeronautics and Space Administration (NASA) has developed several laser-based remote sensing missions such as the Mars Orbiter Laser Altimeter (MOLA), Shuttle Laser Altimeter (SLA), Geoscience Laser Altimeter System (GLAS), Mercury Laser Altimeter (MLA), Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, and Vegetation Canopy Lidar (VCL). Despite great science returns, the generally low maturity level of the laser itself frequently stymies space-based laser instrumentation. In several cases, this low maturity level has led to on-orbit failures, major program delays, and even program cancellations. The principal issue driving space based laser applications is achieving high optical output energies. Remote sensing from space involves long optical path lengths which usually are also subject to atmospheric absorption. Any high output energy laser design must also address waste heat management and longitudinal mode beating.
Longitudinal mode beating is a phenomenon that leads to excessive intra-cavity intensities in high peak power Q-switched lasers. This phenomenon occurs when the laser cavity allows several longitudinal modes to coexist, which can constructively interfere and produce intensities high enough to damage optics. The repeated intensity “spiking” during mode beating generates micro-burns on the optics and slowly eats away at the weakest optical elements or coatings, severely degrading the long term reliability of the laser. These energy spikes also increase the probability of damage related to contamination. To prevent longitudinal mode beating in Q-switched laser systems, the laser must be designed to stay in a single longitudinal mode. Not only does single mode operation have zero mode beating, it also has the added benefits of reducing energy fluctuations from pulse to pulse and substantially narrower spectral output. A narrow linewidth, single mode laser enables the receiver system to use much narrower optical filters to reduce the background light; which greatly enhances daytime measurement capabilities. The improved signal to noise ratio reduces the laser power requirements, thus resulting in a much smaller and more reliable system. One potential application of this type of laser is Doppler wind measurements which require single longitudinal mode operation.
An elegant solution to longitudinal mode beating is to employ a ring laser architecture. In this configuration, a laser develops a single running wave and eliminates spatial hole burning, which is the primary underlying phenomenon that leads to the development of multiple longitudinal modes in the laser. A non-planar ring oscillator (NPRO) can be constructed in a monolithic unit, which maintains optical alignment and allows compact operation of the ring laser. Monolithic NPRO designs exist for CW operation, and for low energy per shot passively Q-switched operation. However, many applications, including the majority of space-based laser missions to date, require high shot energies.
Pulse operation of a laser requires Q-switching, which periodically disrupts the resonant cavity of the laser itself, allowing the laser medium to achieve high population inversions. All current active Q-switching methods require high voltage circuitry, associated power supplies and added thermal management; all of which are undesirable in space based instruments. Passive Q-switching employs an optical material known as a saturable absorber. This method allows pulsed operation with no additional power or circuitry, and requires very little additional thermal management.
Existing monolithic CW and passive Q-switched NPRO lasers employ end-pumping where the pump light is injected into the laser cavity along the optical axis of the NPRO laser itself at one of the facets of the crystal. This method has limited the output energies of passively Q-switched monolithic NPRO lasers for two reasons. The most important issue is that high power end pumping introduces strong thermal lensing effects in the monolithic unit, which degrades the laser stability and beam quality. The second issue is that suitable single emitter laser diode pump sources are limited in output energy.
Thermal management is a major issue affecting all laser designs, but is especially burdensome to designers of space based laser systems. The vacuum of space eliminates convection as a thermal path and the difference between sunlight and shade is extreme. Substantial waste heat can be produced in a laser due to unabsorbed pump light and inefficiencies in the laser itself. Excessive heat can lead to optical misalignment, changes in optical material characteristics, beam pointing errors, out-gassing of contaminants, and even complete system failure.